1. Field of the Invention
The present invention relates generally to odorization of natural gas, and more specifically to an improved system and method for adding odorant to natural gas flowing in a pipeline.
2. Description of the Prior Art
Natural gas is odorless. Because of its potentially dangerous nature, for many years federal regulations have required the addition of an odorant to natural gas so that it can be detected by smell. Odorants such as tertiary butyl mercaptan (TBM) and various blends of commonly accepted chemicals are in common use in the industry.
The odorants added to natural gas, which are provided in liquid form, are often added to the gas at the location where distribution gas is taken from a main gas pipeline and provided to a distribution pipeline. At this point, the gas pressure is stepped down through a regulator, typically from a pressure of approximately 600 psi or more to a lower pressure of approximately 100 psi or less. The odorants can also be added to gas in the main transmission pipeline.
Odorants used with natural gas are extremely concentrated, so that only a small amount of liquid is needed to odorize a relatively large volume of natural gas. For example, with odorants such as TBM and other blends, it is common to use approximately 0.75 lbs. of liquid odorant to adequately odorize 1,000,000 SCF of natural gas.
Odorants such as TBM and other blends are mildly corrosive, and very noxious. It is important that a correctly measured amount of odorant be added to natural gas; otherwise, various problems will result. For example, over-odorization results in excess odors within the valves, pipes, and other equipment used in natural gas distribution. In addition, too much odorant causes the distinctive odorant smell to be noticeable even after the natural gas is burned. This leads to consumer calls complaining of natural gas leaks, each of which must be responded to by the natural gas distribution company. The expense of such calls, when there is no leak involved, is quite high.
It is also important that the odorant levels not be too low. Safety considerations mandate that a natural gas leak be easily detectable by most people. The proper concentration of odorant within natural gas provides this safety measure, but under-odorization is dangerous because actual leaks may not be detected in time.
Two primary techniques are in current use to provide odorization to natural gas in a main distribution pipeline. One technique involves injecting liquid odorant directly into the pipeline. A high pressure injection pump pumps odorant from a liquid storage tank into a small pipe which empties directly into the main gas pipeline. Because the odorant is so volatile, drops injected into the pipeline immediately disperse and spread throughout the gas in the pipeline. Within a few seconds, a few drops of liquid odorant are evenly distributed in gaseous form.
Flow of gas in the pipeline is metered, so that liquid odorant can be injected periodically. Typically, for example, a few drops of odorant is all that is required for a 1,000 SCF of natural gas. When the gas flowmeter indicates that 1,000 SCF of natural gas have flowed through the pipe, the corresponding, pre-calculated amount of liquid odorant is injected into the pipeline. Every time another 1,000 SCF of gas flows past the injection point, another injection is made. Even though the injection is periodic, odorant diffusion within the gas provides for adequately, and relatively even, odorant levels throughout the pipeline.
The injection technique has several important drawbacks. First, as described above, the liquid is extremely noxious. The pump must be designed so that no odorant can leak out. This requires a pump design which is relatively expensive and complex in order to stand up to operating conditions. Failure of this relatively complex injection pump results in the failure of the odorization system.
The second technique for odorizing natural gas involves bypassing a small amount of natural gas, at a slightly higher pressure than the low pressure distribution pipeline, through a tank containing liquid odorant. This bypass gas absorbs relatively high concentrations of the odorant while in the tank. When this heavily odorized bypass gas is placed back into the main pipeline, the odorant, now in gas form, diffuses throughout the pipeline in much the same manner as was the case with the liquid injection system.
Because the bypass gas picks up such large amounts of odorant from the liquid in the tank, becoming completely saturated with odorant gas, it is necessary that carefully monitored small amounts of bypass gas be used. The present approach is to allow a small amount of bypass gas to flow into a holding bottle having a known volume. The bypass line is then closed, and the gas in the bottle is allowed to flow into the odorant tank. This displaces an equivalent amount of saturated bypass gas already in the tank, which then travels into the distribution pipeline.
This bypass technique avoids the failures which can occur with the odorant pumps of the injection technique, but has drawbacks of its own. The valving used to pipe natural gas into the measuring bottle can fail, although this is less likely than failure of an injection pump. However, the bottle is of a fixed size, and cannot easily accommodate large changes in the rate of gas flow through the distribution pipeline. For example, if the bypass bottle was correctly sized for a gas flow of 100,000 SCF per hour, increasing flow in the pipeline to 500,000 SCF per hour can cause difficulties with this technique. Because the size of the bottle cannot change, it must be replaced, or operated five times as often for the higher gas flow. Depending upon the design of the system, this may not be physically possible. For example, if the bottle is operated once every four seconds at the low flow rate, and has a one second cycle time, it is not possible to operate the same equipment at a rate more often than once per second. This situation would necessitate changes to the equipment in the field.
It would be desirable to provide an odorization technique, and equipment to be used therewith, which provides both a more reliable and flexible technique for odorizing natural gas. It would be desirable that the equipment be simple and reliable in operation, and that it be flexible enough to easily accommodate significant changes in the operating condition of the gas pipeline.